Crystallographic characteristics such as orientation and defects are important properties of a polycrystalline material. If known, the crystal characteristics of a material can be very useful to scientists, engineers, and others who use the material for practical applications. For this reason, several techniques have been developed to determine the crystal orientation and other characteristics of a material specimen.
Orientation imaging microscopy (OIM) is one technique that may be utilized to determine the crystal orientation of a polycrystalline material. Generally, OIM involves directing an electron beam at the surface of a specimen. The crystals within the specimen cause the electron beam to form diffraction patterns, which are collected on an imaging or video screen. Such patterns are typically called electron backscatter diffraction patterns (EBSPs). The EBSPs are processed to generate maps that represent the crystal orientation of the specimen. These maps contain important information related to the microstructure of the material.
Although conventional OIM may be adequate for many applications, the usefulness of the technique may be limited by the imaging or measuring equipment. For example, conventional OIM is performed with a scanning electron microscope (SEM), which has a practical resolution of about 400-500 angstroms. While the SEM resolution may be satisfactory for some applications, other applications may require resolution beyond the practical capability of a SEM.
Another known technique for measuring the crystal orientation of a specimen utilizes a transmission electron microscope (TEM) to generate diffraction patterns. Diffraction patterns may appear as bright concentric rings, which are caused by the diffraction of a relatively wide electron beam by many crystals, or as a number of equiradial bright spots, which are caused by the diffraction of a relatively narrow electron beam by a single crystal. The conventional TEM-based crystal orientation technique analyzes the diffraction patterns for individual crystals to determine their specific orientation within the specimen.
A useful and practical crystal orientation map typically includes information related to hundreds of individual crystals. Such a map may reveal information related to microstructure, grain boundaries, and other characteristics that cannot be revealed by the orientation of an individual crystal or a small group of crystals. The conventional TEM-based technique can be painstakingly time consuming if a large number of individual crystals must be analyzed and the results combined to generate a useful orientation map. Even if such a process were automated, the serial acquisition and manipulation of data may be undesirably slow for practical purposes.